Thermoplastic materials are commonly polyolefins such as polyethylene and polypropylene and become pliable or mouldable above a specific temperature, and return to a solid state upon cooling. They can withstand multiple occasions of heating and re-shaping by extrusion, injection moulding, blow moulding, and rotational moulding methods, which makes them recyclable.
Polyethylene and polypropylene possess many desirable characteristics including those of inertness to most chemicals and solvents at ordinary temperature, toughness and flexibility. They are generally poor conductors of electricity.
Thermosetting plastics, on the other hand, are polymer materials that cure by forming irreversible chemical bonds with curing being promoted using heat; a chemical reaction; or irradiation. Thermosetting materials are either moulded into their final form, or used as adhesives. Many thermosetting plastics are foamed as they cure such as the well known polyurethane foam material.
In applications in which a robust impact and wear resistant plastic shell is filled with a foam plastic thermal insulating material, such as in the instance of cold boxes and delivery containers for refrigerated or frozen goods, the most cost effective physical properties for a plastic shell are typically provided by polyolefin plastics which may be any one of a variety of different grades of polyethylene and polypropylene. The shell is typically made by rotational moulding. The rotational moulding process is well suited to the manufacture of double-walled containers, particularly deep containers, such as insulated cold boxes and delivery containers. Such a rotation moulding process typically involves four stages, namely charging the mould with thermoplastic powder; heating and fusing the powder whilst rotating the mould about two axes; cooling the mould during continued rotation; and de-moulding.
Polyolefin plastics, and in particular the polyethylene group that includes those known as linear low density polyethylene (LLDPE), low-density polyethylene (LDPE), medium density polyethylene (MDPE) and high-density polyethylene (HDPE) reportedly dominate the market for rotationally moulded parts with LLDPE reportedly being the most popular by an appreciable margin.
While the polyolefin plastics generally have good impact and chemical resistance, the properties of strength and stiffness of LLDPE is known to be rather temperature dependent. As a result and especially coupled with a high coefficient of expansion, an LLDPE rotational moulding may show some distortion or bulge when exposed to intense sunshine or other heat source. LLDPE is inherently flexible and possesses limited stiffness that has led mould designers to use an array of features such as indentations, ribbing, angles and corrugations to add rigidity and strength to a moulded product.
The filling of hollow double-walled containers with plastic foam is a cost effective method of adding stiffness, strength and rigidity to a rotational moulded product. The foam core provides the additional benefits of shape retention as well as excellent thermal insulation and buoyancy. Polyurethane foam is frequently the foam-type of choice for post-moulding applications. It is a low density, rigid, heat insulating foam that has exceptional thermal properties and possesses a low coefficient of thermal expansion. Polyurethane foam may be applied to a cavity to be filled in a variety of different ways that include the use of a measured and mixed, two-part liquid form which expands into a mass of bubbles and cures quickly.
Unfortunately, the desirable properties of LLDPE as an outer skin and the desirable properties of polyurethane foam as a general use, wide application heat insulating filler do lead to difficulties in causing the one to attach to the other to an adequate extent. This phenomenon is attributed to the fact that the polyurethane foam tends to form a smooth skin at the interface between its cellular core structure and the LLDPE substrate as it does when such a foam is cured directly in a mould treated with a release agent.
One problem associated with inadequate attachment is that the outer skin or shell can move independently of the foam core and due to their varying coefficients of thermal expansion, expansion of the shell can take place independently of the foam. From a structural point of view there is no way in which a shear force can be reliably transferred from the shell to the foam as would be desirable in a sound sandwich construction composite.
Whilst polyurethane foam, a popular thermosetting filler material, may have generally high levels of attachment with respect to a wide spectrum of substrates and other plastic materials, it is not compatible with LLDPE and its bond with this thermoplastic is very weak. Bad attachment and incompatibility of polyethylene with other materials such as pigments, paints, and other polymers has earned it the classification of a ‘hard-to-bond material’.
It is common practice in the plastics industry that hard-to-bond materials be subjected to surface modification (treatment or preparation) operations to enhance their attachment potential. Current surface modification methods include heat, chemical, priming and physical treatments. All are designed to increase the reactivity and roughness of the surface of the plastic. However, preparing the internal surface of rotational moulded products for polyurethane foaming by such post-moulding secondary operations gives rise to challenges in terms of associated extra costs, adaptability, practical implementation, convenience and safety.
In instances in which the surface to be modified is suitably accessible, surface modification methods include fluorination using fluorine gas with short exposures with the disadvantage that fluorine gas is highly toxic and corrosive and can be fatal if inhaled; corona discharge and plasma treatment with an accompanying sizeable investment in capital equipment or the outsourcing of treatment; flame treatment that is mostly used on polyolefin sheet materials and smaller simple shapes of items; primers in which a reactive species is dissolved and applied to a surface using a brush or spray; chemical etching such as chromic acid etching which can be hazardous and results in its use on a limited basis; and surface roughening that can be accomplished by abrasion, grit blasting and etching but has accompanying dust and health hazards.
Various other methods of surface modification are more appropriate to an enclosed surface such as the inner surface of a rotational moulding, and may involve charging the mould with a combination of powder grades to promote an oxidized internal skin to the thermoplastic shell in order to encourage adequate attachment to a filling of thermosetting plastic that is typically polyurethane foam. However, there may be a likelihood of producing a moulding with an impaired outer skin or weakened structure.
The addition of other particles of non polymer grit such as steel, aluminium, ground nut shell and other foreign materials to the rotomoulding cycle with the polymer charge is also a possibility. The particles become imbedded in the LDPE substrate and protrude out of it to present a keyed surface for attachment to the plastic foam. The downside of this method is the likelihood of particles protruding through the good surface and impairing same and the embedded foreign particles may weaken the LLDPE skin particularly where there is a concentration of foreign matter.
Another technique is for the moulding process to be stopped short of fully melting the LLDPE powder charge in the mould so that the powder is not fully melted at the inner surface which remains rough and porous and offers better mechanical grip for improved polyurethane foam attachment. This technique is difficult to control because rotational moulding is affected by many factors including ambient temperature. An excessively “under-cooked” product will have impaired strength and may be rejected. A perfectly “under-cooked” product may require an excessive charge size and therefore the product may cost and weigh more than necessary.
A more recent technique is to charge the mould with plasma treated powders and granules to mould an item with an attachment enhanced inner surface. The technology required to undertake the plasma treatment is confined to a few exclusive intellectual property owners and equipment manufacturers
There is a need to provide an enhanced method of bonding different plastics materials to each other such as in the case of a thermoplastic to a thermosetting plastic. There is also a need to provide a method of rotational moulding whereby an attachment promoting agent of the two different plastics may be introduced into the moulded item at a specific stage during the moulding cycle.